[bug] Unsubscribe requests are not sent when RSU exits

added previous PR comments feedback.

gupnp_service_proxy_set_subscribed(FALSE) is called each time we call gupnp_service_proxy_remove_notify(), except for the subscription lost case where  unsubscription is implicit. (checked with Régis)


when RSU exits, send first the requests to unsubscribe
for device contexts notification, and delay the g_main_loop exit
for 1 second to give them a chance to be really sent.

Fix issue: https://github.com/01org/renderer-service-upnp/issues/92

Signed-off-by: Christophe Guiraud <christophe.guiraud@intel.com>